WASHINGTON (Reuters) - About 9 percent of U.S. troops who
have performed combat duty in Iraq and Afghanistan report
experiencing post-traumatic stress disorder, according to a
study led by Defense Department researchers.

The study, published on Tuesday in the British Medical
Journal, provides the latest estimate of this mental disorder
among U.S. troops returning from those wars.

It tracked PTSD among about 50,000 U.S. troops, looking at
differences in rates among those who saw combat, those who
deployed in noncombat roles and those who did not deploy.

Some troops develop it after wartime trauma like suffering
wounds or witnessing others being hurt. It often appears months
or years after a traumatic event.

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"The unpredictability and intensity of urban combat,
constant risk of roadside bombs, multiple and prolonged tours,
and complex problems of differentiating enemies from allies can
leave many troops with high stress levels and possible lasting
health consequences," the researchers wrote.

The troops who participated in the study were first
questioned about their health from 2001 to 2003 before
deployment, then questioned again from 2004 to 2006.

The researchers found that 8.7 percent of those who were
deployed into combat who never previously had PTSD reported
experiencing symptoms of it or being diagnosed with it. That
compares to 2.1 percent of troops who were deployed but did not
see combat and 3 percent of those not deployed.

The study was led by Tyler Smith of the Department of
Defense Center for Deployment Health Research at the Naval
Health Research Center in San Diego.

Experts say PTSD symptoms include irritability or outbursts
of anger, sleep difficulties, trouble concentrating, extreme
vigilance and an exaggerated startle response. A person may
initially respond to the trauma with horror or helplessness,
then may persistently relive the event.

Some U.S. lawmakers and outside experts have expressed
concern that the Pentagon has not adequately provided for the
mental health needs of troops returning from war. The Pentagon
has defended its performance.

The Institute of Medicine, which advises U.S. policymakers
on medical issues, has said that other research indicated about
12.6 percent of troops who have served in Iraq and 6.2 percent
who served in Afghanistan have experienced PTSD.